The present invention pertains to the art of oxygen concentration, and more particularly to the art of concentrating oxygen from a gaseous mixture, such as air, according to an electrochemical device and method. The invention finds usefulness in a variety of applications and situations such as those pertaining to the medical, industrial and space exploration fields, and will be described with particular reference thereto. It should be appreciated, however, that the invention can be applied to other areas not set forth herein, particularly where concentrated oxygen is required.
There is a need for providing substantially undiluted oxygen in a variety of medical, industrial, aeronautical and space exploration situations. Numerous electrochemical methods and devices are known in the art for concentrating oxygen, but these are not without several disadvantages. Such known electrochemical devices and methods for generating oxygen are based on electrolysis using solid polymer electrolyte (SPE) technology, static feed H.sub.2 O electrolysis subsystems (SFWES), or the less- developed electrolysis of carbon dioxide. Conventional electrochemical oxygen generators are based upon the electrolysis of water, a 4-electron process.
Existing solid polymer electrolyte (SPE) technology includes a thin (0.30 mm) perfluorinated sulfonic acid membrane (DuPont Nafion) which, when saturated with water, serves as an electrolyte having a resistivity of 15 ohm cm. The membrane also prevents mixing of O.sub.2 and H.sub.2. Catalyzed electrodes are placed in intimate contact with both sides of the membrane. Deionized water is fed to the cathode (i.e., the H.sub.2 producing side of the SPE) and acts as both a reactant and a coolant. A six person life support SPE electrolyzer for manned space flight, which operates at 1.72 V per cell at a current density of 350 mA/cm.sup.2, calls for an input power of 2180 W and provides an O.sub.2 production rate of 6.82 kg/day. The specific power is roughly 320 W/kg O.sub.2 per day.
Existing static feed water electrolysis subsystem technology (SFWES) includes thin asbestos sheets. These serve as both the water feed and the cell matrices, and are saturated with an aqueous KOH solution. As a direct current power is supplied, water in the cell matrix is electrolyzed. As a result, the KOH electrolyte concentration increases and water vapor diffuses from the feed to the cell matrix. An SFWES module which operates at 1.52 V per cell at a current density of 206 mA/cm.sup.2, calls for a power consumption of 174 W, and provides a O.sub.2 production rate of 0.82 kg/day. The specific power for such an operation is roughly 212 W/kg O.sub.2 per day.
Another preexisting oxygen concentration method involves carbon dioxide electrolysis. An amount of CO.sub.2 from a CO.sub.2 concentrator is directly oxidized using a solid oxide electrolyte that is coated on both sides with a porous metal coating such as Pt. This operation takes place at high temperatures to produce O.sub.2. Only oxide ions (O.sup.2-) migrate through the solid electrolyte driven by a DC voltage so that O.sub.2 separation is excellent. The processes are as follows:
______________________________________ At cathode: CO.sub.2 + 2e .fwdarw. CO + O.sup.2- At anode: 2O.sup.2- .fwdarw. O.sub.2 + 4e In another reactor: 2CO .fwdarw. C + CO.sub.2 ______________________________________
Technological problems such as sealing exist with carbon dioxide electrolysis.
Other electrolysis technologies are known in the art. One example concerns the electrolysis of water. Electrolysis of water with O.sub.2 -depolarization utilizes a fuel cell type cathode and operates at a theoretical cell voltage close to zero. In practice, overpotentials at the anode and cathode, as well as IR losses, raise the cell voltage to about 1.1 V, at current densities of 108 mA/cm.sup.2. The specific power requirement is about 180-200 W/kg O.sub.2 per day.
Another technology involves electroregeneration of an organometallic carrier compound capable of binding O.sub.2 (in a manner like hemoglobin) in its reduced state and releasing O.sub.2 according to a 2-electron process upon anodic oxidation is being studied. A major parasitic reaction with respect to power consumption arises from the oxidation of a carrier which has not bound to O.sub.2. The power consumption for such technology is estimated at about 30 W/kg O.sub.2 per day. A limitation is the low current density of practical operation, in the range of about 1 to 2 mA/cm.sup.2. It should be noted that at higher current densities, the lifetime of the organometallic is severely limited.
The electrogeneration of reducing agent 2,7-anthraquinone-disulfonate in solution reduces O.sub.2 to peroxide, which is then electrochemically oxidized to O.sub.2. This 2-electron process suffers from high solution IR and low energy efficiency.
Direct electroreduction of O.sub.2 can produce the superoxide ion (O.sub.2.sup.-), which may be followed by diffusion to the anode and one- electron oxidation to O.sub.2 at low power requirement. The key problem here is stabilization of the superoxide ion in water and avoiding coelectrolysis of water to give H.sub.2 and O.sub.2.
The present invention pertains to an electrochemical device and method for the selective removal and regeneration of oxygen from the ambient atmosphere. It is based on the use of a two gas-fed electrode system separated by a thin layer of liquid or solid electrolyte. An external potential difference is applied between the two electrodes to promote the reduction of dioxygen to hydrogen peroxide at the cathode. The hydrogen peroxide is then reoxidized at the anode to yield gas-phase purified dioxygen. Hence, if the cathode is exposed to the atmosphere and the anode is exposed to an enclosed environment, the device will selectively enrich the enclosed environment with oxygen.
The subject development is well suited for applications in areas where oxygen is continuously consumed in or by the enclosed environment. A difference in chemical potential or partial pressure between the atmosphere and the enclosed environment will decrease the energy requirements needed for driving the process to reasonable rates, thus providing an energy efficient and economical source for purified oxygen.
The subject new electrochemical filter and method for its application may lead to inexpensive sources of pure oxygen for medical uses and, on a larger scale, for industrial applications such as steel production or glass manufacturing.